An image forming system is configured, for example, by connecting an image forming apparatus and a terminal device via a network. Normally, a user performs a print request operation at the terminal device. Printing data are then transmitted from the terminal device, and upon receiving the printing data, the image forming apparatus performs, as a single job, an expansion process on the printing data and output of a sheet on which is printed an image that is in accordance with the expanded data. Among the printing data transmitted from the terminal device, there are materials of a high level of confidentiality that should be prevented from being seen by others.